The present invention describes a method for receiving a signal in device-to-device communication.
Prior to description of the present invention, device-to-device communication is described. FIG. 1 is a conceptual diagram of device-to-device communication.
Referring to FIG. 1, UE1 120 and UE2 130 perform device-to-device communication. Here, a UE refers to a terminal of a user. However, network equipment such as a base station can be regarded as a UE when the network equipment transmits and receives signals using device-to-device communication. An eNB can control positions of time/frequency resources, transmit power and the like for device-to-device communication between UEs using appropriate control signals. However, when UEs are located outside the coverage of an eNB 110, device-to-device communication can be performed without a control signal of the eNB 110. In the following description, device-to-device communication is referred to as D2D communication, a link for D2D communication is referred to as a D2D link and a link through which a UE communicates with an eNB is referred to as an NU (eNB-UE) link.
The UEs linked to the eNB 110 perform D2D communication using subframes and perform communication with the eNB using other subframes. When such D2D system is designed, it is necessary to consider timing advance provided to uplink transmission of UE1 120 and UE2 130 and propagation delay between UE1 120 and UE2 130. When UE1 120 transmits a signal to UE2 130, the boundary of a D2D subframe received by UE2 130 may not correspond to a subframe boundary of UE1 120 and a subframe boundary of UE2 130 due to propagation delay generated during transmission. Since D2D communication is generally performed between UEs located close to each other, it is possible to assume that D2D communication is performed with round-trip propagation delay within 0.5 orthogonal frequency division multiplexing (OFDM) symbol. When a UE transmits a signal to the eNB in a specific subframe and receives a D2D signal in the subsequent subframe, a predetermined time is required for switching from transmission operation to reception operation. Furthermore, even when the UE performs the reception operation and then transmits a signal to the eNB in the subsequent subframe, a predetermined time is required for switching from reception operation to transmission operation. That is, a predetermined guard interval (referred to as GI hereinafter) is present between a D2D link subframe and an NU link subframe for cancellation of propagation delay between UEs and transmission/reception operation switching, and transmission and reception of significant signals need to be suspended in the GI. In general, the GI is shorter than the duration of a single OFDM symbol and, particularly, can be regarded as being less than 0.5 symbol duration.
When the GI is set to be less than 0.5 symbol length, a special signal can be transmitted using the remaining 0.5 symbol duration of one symbol. The special signal can be used to detect a correct time at which a D2D subframe starts by using a sequence known to a D2D receiver UE. In view of this, the special signal can be called a D2D subframe synchronization signal or a D2D subframe determining signal. The special signal may be used for channel estimation for D2D signals and measurement for channel state information (CSI) feedback since the special signal is known to a D2D receiver.